<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .a {
            width: 100px;
            height: 50px;
            background-color: red;
            position: fixed;
            text-align: center;
            top: 100px;
            display: flex;
            justify-content: center;
            align-items: center;
        }

        .flex {
            display: flex;
            justify-content: center;
        }

        .all {
            height: 100vh;
            background-color: black;
        }
    </style>
</head>
<body>
    <div class="all"></div>
    <div class="flex">
        <div class="a">
            hello world
        </div>
    </div>
</body>
</html>